This invention relates to marine seismic prospecting and, more particularly, to a connection system for attaching equipment to seismic streamers.
A marine seismic streamer is a cable, typically several thousand meters long, that contains arrays of hydrophones and associated electronic equipment along its length. One purpose of the streamer is to position the hydrophone array at a known depth and position relative to a towing vessel in a survey area. Externally mounted equipment, such as depth controllers, called “birds,” lateral-position controllers, emergency recovery pods, and acoustic pods, performs the functions of positioning and controlling the cable.
Individual external devices are attached to the streamer at various positions along its length. In one conventional streamer connection system, a collar arrangement uses a hinge and latch mechanism for operation. Examples of this kind of system are described in U.S. Pat. No. 5,507,243, “Connector For Underwater Cables,” Apr. 16, 1996, to Oneil J. Williams et al. and in U.S. Pat. No. 5,709,497, “Latching Device,” Jan. 20, 1998, to David W. Zoch et al. Another conventional connection system uses C-shaped collars that slip radially onto the cable and slide axially into position. Examples of this system are described in U.S. Pat. No. 6,263,823, “Connection System for Connecting Equipment to Underwater Cables,” Jul. 24, 2001, to André W. Olivier. The C-shaped collars dispense with the need for hinges and latches. All these kinds of outer collars, with external devices attached, are attached around inner collars affixed to the cable as the cable is payed out from the back deck of a survey vessel. Circular cylindrical inner surfaces on the outer collars ride on races formed on the exterior of the inner collars. This allows the streamer to rotate inside the outer collars without also rotating the external devices.
In a typical arrangement, an external device is connected to the streamer at both a fore and an aft position. In most circumstances, the stiffness of the streamer under the tension of the tow maintains the section of streamer between the fore and aft positions straight. In the case of a bird or other external device having a heading sensor referenced to the center line passing through the centers of the two outer collars, maintaining a parallel relationship between the streamer's axis and the collars' center line is important in deriving an accurate indication of the heading of the streamer at the location of the sensor. But, at slow tow speeds when streamer tension drops or in the face of high lateral forces caused by external lateral steering devices, the section of streamer between the fore and aft collars can bend and twist out of a straight alignment. The action of the streamer can also force the fore and aft collars out of alignment, their centerline offset or angled relative to the streamer's axis. This misalignment causes uncorrectable errors in the heading sensor reading. Furthermore, the twisting and bending of the streamer can cause the inner collars to bind in the outer collars, which hinders the free rotation of the cable within the outer collars relative to the attached external device, whose performance is consequently compromised.